Long-lasting, activity-dependent changes in the strength of synaptic transmission play a critical role in the development of neural circuits and in the storage of information. Such changes also appear to play an important role in the recovery of the brain from a variety of pathological insults. The most compelling and extensively studied model for such changes has been that form of long-term potentiation (LTP) observed in hippocampal CAl pyramidal cells. It is clear, however, that there are many forms of synaptic plasticity and that their underlying mechanisms differ. The primary goal of this project is to elucidate the molecular mechanisms underlying the novel form of LTP observed at the synapses between mossy fibers and hippocampal CA3 pyramidal cells (MF LTP). Unlike the LTP in CAl cells, MF LTP does not require activation of NMDA receptors but appears to be induced by presynaptic activation of the cAMP-dependent protein kinase (PKA) resulting in a long-lasting increase in neurotransmitter release. A number of different physiological and biochemical approaches will be used to examine MF LTP. Because MF LTP can be generated in single cell cultures of dentate granule cells, it will be possible to directly monitor changes in synaptic vesicle exocytosis and endocytosis using the fluorescent dye FMl-43. Biochemical assays will be used to monitor the time course of changes in cAMP levels and PKA activity during MF LIP as well as to determine whether specific presynaptic phosphoproteins, in particular rabphilin 3A, may be involved in this form of plasticity. A complementary set of experiments will examine MF LTP in a line of mutant mice which is lacking the specific presynaptic protein, rab3A. Because neurotrophins appear to play an important role in experience-dependent cortical plasticity and also cause long-lasting increases in neurotransmitter release, their synaptic actions will be examined and compared to MF LTP. An examination and comparison of MF LIP and the synaptic actions of neurotrophins will markedly enhance our understanding of the basic mechanisms of synaptic plasticity in the mammalian brain. This in turn will facilitate the development of interventions that will either prevent or promote recovery from the pathological insults accompanying a number of neurologic disorders such as stroke and epilepsy.